To increase the availability of data networks in vital or otherwise critical applications such as vehicle control, drive control or substation automation, redundancy is required. Basically, the latter may be provided by duplicating the communication lines, e.g. the electrical or optical transmission lines, as well as at least the physical layer of the corresponding protocol stack. While the sending of duplicate information generally does not represent a major obstacle to a straightforward implementation, receiving and handling of duplicated information may require special provisions.
The communication standard IEC 61375, for the purpose of improving availability to enable its use in rail vehicles, specifies two physically independent elementary networks and introduces a frame-by-frame redundancy, in which a sender sends redundant frames simultaneously over the two lines, and a receiver primarily receives the data over one line while observing the other. US Patent Application Publication US 2003/0147377 A1 applies this principle to redundant switched full-duplex Ethernet type communication networks. For the purpose of filtering duplicates, the sender adds an identifier field to the frames before sending identical frames on both elementary networks. A receiver accepts a frame only if its identifier has not been received previously via the other network, i.e. duplicates are rejected at the link layer. The identifier is a continuous frame number or sequence counter value of successive frames that is restarted from zero after a determined count as disclosed e.g. by the IEEE standard 802.2. A given time window delimits the time for acceptance of a frame. In order to maintain the standard Internet Protocol (IP) frame header, the sequence counter can be inserted between the payload and the check sequence.
The aforementioned method works well within a closed environment in which all devices are expected to insert a sequence counter in the frames. However, in particular in the area of substation automation, third-party devices and/or mobile devices such as engineering laptops are often connected to one of the two redundant lines only and do not have knowledge of the particular redundancy protocol used by the majority of devices in the network. In this case, the receivers are confronted both with frames that carry a sequence counter and frames that do not carry a sequence counter. In this case, the receivers might either interpret the field comprising the sequence counter and appended to the former frames as an error or ignore it as padding. While the TCP and UDP protocols are able to handle this case, other protocols could actually reject the frame. To avoid this, extensive network and device configuration procedures determining for each device if it does or not obey to the redundancy protocol would be required prior to any data communication. In addition, a general problem of Ethernet frames resides in the fact that in some cases, there is no reliable information about the frame length. Therefore, truncating a frame could result in a correct checksum in some cases, leading to an undetected error.
EP 854 610 discloses an Ethernet redundancy method without switch-over for exchanging measurement data and control commands. The method discards duplicates (i.e. redundant frames) at a low level control section of the receiving device by means of a data identifier, but does not provide for an active detection of singular frames deprived of said data identifier. In detail, singular transmission data addressed to a receiving device is stored in a reception area of a buffer section. In the subsequent step of checking if identical data has already been received over the redundant system's communication line, no such identical data is found, and the singular data is actually forwarded to the respective reception queue as well as to the upper level application. After some time, the abnormality process removes the singular data from the reception queue, as for these no duplicate will be received within a predetermined timeout period. The occurrence of singular data is treated as a scarce case of abnormal behavior, and the method inherently processes the singular data to the upper level application without even considering to identify the singular data at the low level control section of the receiving device.